1. Field of the Invention
The present invention relates to a light emitting element driving apparatus, a method of driving a light emitting element and a computer readable recording medium storing a computer program for driving a light emitting element. The light emitting element driving apparatus executes a pulse width modulation (PWM) driving operation at a predetermined duty cycle and further executes a soft-start driving operation during each ON-period in the PWM driving operation. During the soft-start driving operation, current to be supplied to the light emitting element is increased from a first constant current value up to a second constant current value.
2. Description of the Related Art
Light-Emitting Diodes (LEDs) are widely used for various purposes. For example, in battery-driven mobile devices such as cellular phones and PDAs (Personal Data Assistants), an LED or a light-emitting device is used as a backlight for an LCD (Liquid Crystal Display), and an LED is also used as a flash for a CCD (Charge-Coupled Device) camera built in such a mobile device. LEDs having different luminescent colors are also driven in a flashing manner to be used for various purposes.
Various techniques are used for driving an LED with a constant current to make the LED emit light stably. For example, a pulse width modulation (PWM) driving technique has been proposed for supplying current to an LED to enable stable light emission. The PWM driving technique drives the LED by repeating a first period Ton, in which the LED is turned on, and a second period Toff, in which the LED is turned off, with a duty cycle Dt (e.g., 1 ms.), which is faster than human recognition, as shown in FIG. 8A.
Meanwhile, a technique called a “Soft-Start” driving operation has been proposed for preventing inrush current. In the “Soft-Start” driving operation, current is supplied to the LED during a predetermined period Ta with a first constant current value A1 and the current increases to a second current value A2, as shown in FIG. 8B.
For example, a voltage booster type circuit, which executes such a “Soft-Start” driving operation is disclosed in Japanese Non-examined Patent Publication No. 2005-160178.
A voltage booster type circuit, such as the one disclosed in JP 2005-160178, often has a minimum current value of current that should be supplied to the LED for stable driving. For example, as shown in FIG. 9, fluctuation of current supplied to the LED due to environmental temperature changes rapidly increases in a lower range of input current value. Therefore, it is preferable to supply a current to the LED that has a current value that is above such a high fluctuation range (i.e., that has a current value in a stable range of current values). Hereinafter, a current value that is a lower limit of the stable range is referred to as a “Minimum Value”.
On the other hand, a current value is previously determined for current to be supplied to the LED for obtaining desirable light-emitting luminance. If the LED is driven by using a time-division driving technique such as the PWM driving technique and the mean amount of the current supplied to the LED is constant in each duty cycle, it is known that the average light-emitting luminance is almost the same even if the duty cycle ratio of the PWM driving technique changes. The “Rated Value” of current to be supplied to the LED is defined in terms of the mean value of current that is supplied to the LED to obtain desirable light-emitting luminance. That is, the “Rated Value” is a mean value of the current that must be supplied to the LED to achieve the desirable light-emitting luminance. Under such a definition, the “Minimum Value” often becomes larger than the “Rated Value”.
In the “Soft-Start” driving operation, a transition period Tb is needed, during which the current value of the current supplied to the LED increases from the first constant current value A1 to the second current value A2. Including the period Ta, during which the LED is controlled with the first constant current value A1, a time period Tc of about hundreds of microseconds is needed to increase the current supplied to the LED up to the second current value A2.
Therefore, when executing the PWM driving technique, when the duty cycle is set to a relatively short time period, and when also executing the “Soft-Start” driving operation in the turned-on period Ton, the ratio of the time period Tc to the turned-on period Ton increases. That is, since the time period Tc is required to increase the current value to the second current value A2, a time period Td during which the second current value A2 is stably supplied to the LED becomes extremely short, as shown in FIG. 10. As a result, lighting of the LED becomes unstable.